Is 72 year old Republican John McCain’s cognitive ability showing signs of decline?

You betcha!:

Dr. Robert Epstein, former editor-in-chief of Psychology Today, on aging and cognitive abilities:

[S]everal studies conducted over the past decade or so show that when it comes to individuals, brain size is in fact an excellent predictor of a variety of cognitive abilities. Does brain size follow the pattern we see with intelligence and memory? Indeed it does. A recent MRI study conduced by Eric Courchesne and his colleagues at the University of California San Diego shows that brain size in humans peaks at about age 14 and declines gradually thereafter. By the time a man–such as candidate McCain — is 70, his brain has shrunk to the size it was when he was about 3. This pattern occurs both for overall brain volume and for the all-important gray matter that contains signaling neurons.